Abstract
It is difficult to study “hedonic” or “pleasant” emotion with nonhuman animals. We have to deal with objectively observable behavior. In this regard, we investigated neural mechanisms of reward seeking. In a series of experiments, we have shown that（1）the endogenous nociceptin modulates preference for high fat diet,（2）the theta rhythm in the hippocampus possibly relates to prediction of reward,（3） dopamine receptors in the hippocampus increased along with the formation of reward memory, and （4）dopamine receptors in the amygdala play an important role in the behavioral expression of preference. These results indicate the significance of learning and memory in reward seeking. Although reward seeking is an essential biological function for survival, the very same system is also responsible for the pathological states such as addiction and dependence. We must further study and clarify both natural and aberrant aspects of reward seeking. The revealed nature of which will then sheds light on the structure and function of “hedonic” or “pleasant” emotion.
